*3.2. Microhardness, Modulus of Elasticity, Residual Stress and Adhesion Strength of DLCs*

Table 6 shows the results of hardness measurements according to the Martens scale (HM), which characterizes both elastic and plastic properties of DLC-coated carbide samples. As follows from the experimental data obtained, with an increase in the applied load and, accordingly, with an increase in the penetration depth of the indenter, the hardness of the surface layer decreases. Within one load (1.0 mN), the hardness of the coated samples varies from 32 to 38 GPa, and from 23 to 29 GPa at a load of 4.0 mN, respectively. Simultaneously, the microhardness value is practically identical for samples with DLCs without a sublayer and with a (CrAlSi)N sublayer. Another type of change is observed when evaluating the modulus of elasticity of samples with DLCs. If at a load of 1.0 mN, the modulus of elasticity of the two types of samples had similar values, 284–300 GPa for samples with DLC without sublayer and 275–295 GPa for samples with (CrAlSi)N/DLC-coated, then with an increase in the load to 4.0 mN, significant differences were observed of 235–239 and 193–209 GPa, respectively. The data in Table 6 also gives an idea of the index (coefficient) of the plasticity of the studied samples. At an applied load of 1.0 mN, the index of the plasticity of the samples are very close and is 0.113 for samples without sublayer and 0.119 for samples with (CrAlSi)N/DLC, and with a four-fold increase in the load to 4.0 mN, the differences are quite noticeable, 0.10 and 0.129 respectively.


**Table 6.** Physical and mechanical characteristics of DLCs deposited on hard alloy samples.

It should be noted that the obtained value correlates to the known data of other research groups and show that the researched DLC is not fragile since the measured range of values for hardness is relatively low when sublayer increases its plasticity.

As follows from the results of the analysis of samples on an X-ray diffractometer (Figure 6), the character of the distribution of residual compressive stresses in the coating for samples with a

single-layer DLC and a (CrAlSi)N/DLC do not change significantly. Attention is drawn to a noticeable decrease in residual stresses' average values when a DLC is applied to a sample with a (CrAlSi)N sublayer. Average residual stresses for single-layer DLCs are 1250–3650 MPa, while 650–2750 MPa for DLCs with a sublayer.

**Figure 6.** Values of residual stresses in DLCs deposited on different samples: (**a**) coating without sublayer and (**b**) coating with sublayer (CrAlSi)N.

Figure 7 shows optical images of indenter movement tracks on the surface of hard alloy samples with DLCs and their corresponding loads from 1 to 30 N obtained from scratch testing. The presented panoramas make it possible to quantify the value of critical loads at which the coatings break off from the carbide substrate and on which part of the indenter movement path this occurred. Acoustic emission sensors registered peak loads indicating the beginning of the indenter penetration into the coating (*P*1), the beginning of the first crack (*P*2), the peeling of local sections of the coating from the substrate (*P*3), and the critical (destructive) load (*P*4). It can be seen that, for samples with (CrAlSi)N/DLC, the peak loads are shifted to the right. In particular, the critical load *P*4 corresponds to significantly higher forces (about 23 N), while for a sample with a DLC without a sublayer the critical load corresponds to the force equal to 12 N. It should be noted that to exclude erroneous experimental results and subsequent erroneous conclusions, five samples of each type of DLCs were subjected to scratch testing. The data obtained on the quantitative values of critical loads for the samples under study were similar, and the spread of values was ±2 N.

**Figure 7.** Tracks of indenter movements on the surface of 6WH10F hard alloy samples with DLCs and corresponding loads from 1 to 30 N; coating without sublayer (upper image), coating with sublayer (CrAlSi) N (lower image).

Observing such significant differences in the results of scratch testing of samples with DLCs, we additionally performed a qualitative rapid assessment of the strength of the adhesive bond by indentation of the Rockwell indenter. Figure 8 shows the micro images of the indenter insertion area in the samples. It is seen that the penetration of an indenter into a sample with a DLC without a sublayer is accompanied by the exfoliation of micro-sections of the coating in a circle at a certain distance from the indentation area (Figure 8a). In this case, the load applied to samples with (CrAlSi)N/DLC (Figure 8b) does not cause noticeable peeling and destruction of the outer coating.

**Figure 8.** Rockwell indenter indentation area in the surface layer of 6WH10F hard alloy samples with DLCs: (**a**) coating without sublayer and (**b**) coating with sublayer (CrAlSi)N.
